cache: make public DMS conversion result cacheable even for authenticated users.

also :
* remove Accept-Encoding from Vary header, because Zope's response does not differ by Accept-Encoding request header.
* use getModificationDate() for modification time if available.

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web/Base_isPublicConvertionResult.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="PythonScript" module="Products.PythonScripts.PythonScript"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>Script_magic</string> </key>
+            <value> <int>3</int> </value>
+        </item>
+        <item>
+            <key> <string>_bind_names</string> </key>
+            <value>
+              <object>
+                <klass>
+                  <global name="NameAssignments" module="Shared.DC.Scripts.Bindings"/>
+                </klass>
+                <tuple/>
+                <state>
+                  <dictionary>
+                    <item>
+                        <key> <string>_asgns</string> </key>
+                        <value>
+                          <dictionary>
+                            <item>
+                                <key> <string>name_container</string> </key>
+                                <value> <string>container</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_context</string> </key>
+                                <value> <string>context</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_m_self</string> </key>
+                                <value> <string>script</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_subpath</string> </key>
+                                <value> <string>traverse_subpath</string> </value>
+                            </item>
+                          </dictionary>
+                        </value>
+                    </item>
+                  </dictionary>
+                </state>
+              </object>
+            </value>
+        </item>
+        <item>
+            <key> <string>_body</string> </key>
+            <value> <string># This script has \'Anonymous\' proxy role to check \'View\' permission for Anonymous.\n
+return \'format\' in context.REQUEST and context.getPortalObject().portal_membership.checkPermission(\'View\', context)\n
+</string> </value>
+        </item>
+        <item>
+            <key> <string>_params</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>_proxy_roles</string> </key>
+            <value>
+              <tuple>
+                <string>Anonymous</string>
+              </tuple>
+            </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>Base_isPublicConvertionResult</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>